Fusible links are often employed in an integrated circuit (IC) to trim one or more of the parameters of the IC. A typical fusible link has an intact state and a blown state. In the intact state, the fusible link provides a very low resistance and in the blown state the fusible link provides an open circuit. One example application of a fusible link is trimming the offset voltage of an IC with fusible links connected in series with resistors which are configured such that the resistance between an amplifier's offset input and ground can be adjusted by blowing open the appropriate links.
A typical prior fusible link system includes a large MOS transistor selector circuit which provides a large current to blow the fusible link element. The large transistor of the fusible link wastes value chip space on the IC.
Another type of fusible link which overcomes the problems associated with the large selector circuit is known as a semi-fusible link. The semi-fusible link has a first resistance in the intact state and a second higher resistance in the blown state (which is not an open circuit). A typical prior art semi-fusible link uses a thin film resistor which reduces the amount of current required to blow the fusible link and therefore reduces the amount of space utilized on the IC. An example of a semi-fusible link is disclosed in U.S. Pat. No. 6,246,243, incorporated herein by reference.
The '243 patent typically includes a semi-fusible link element, a select transistor, active circuitry connected to the semi-fusible link element, and an interconnect layer (e.g., metal one layer) to interconnect to a semi-fusible link element to the semi-fusible link system. However, the design of the '243 patent requires both the semi-fusible link and the active circuitry to be connected to the metal one layer which prevents the semi-fusible link element and active circuitry from being disposed over each other. The design also requires the selector circuit to be on the same layer as the fusible link element. The result is that the fuse link element, the active circuitry and the selector circuit must be laterally disposed on the same layer as the metal one layer which wastes valuable chip space.